RAX-2 is a RAX a follow-on collaborative 3U CubeSat mission comprised of teams from the University of Michigan (UMich), Ann Arbor, MI (James W. Cutler), and SRI International, Menlo Park, CA (Hasan Bahcivan), USA. The payload was developed at SRI, and the satellite bus was developed in the Michigan Exploration Laboratory (MXL). The mission is funded by NSF (National Science Foundation) to study space weather.

RAX is a space weather mission designed to study plasma field-aligned irregularities in the ionosphere. It has enabled undergraduate students, graduate researchers, engineers, and scientists to be involved in the design, building, and operations of two spacecraft, RAX-1 and RAX-2.

Small spacecraft are more highly resource-constrained by mass, power, volume, delivery timelines, and financial cost relative to their larger counterparts. Small spacecraft are operationally challenging because subsystem functions are coupled and constrained by the limited available commodities (e.g. data, energy, and access times to ground resources). Furthermore, additional operational complexities arise because small spacecraft components are physically integrated, which may yield thermal or radio frequency interference.

The project extended the initial MBSE (Model Based Systems Engineering) framework developed for a small spacecraft mission by demonstrating the ability to model different behaviors and scenarios. MBSE and SysML (Systems Modeling Language) were applied to model an actual CubeSat, the RAX (Radio Aurora Explorer) mission. 1)2)

Background: RAX-1 (launch Nov. 20, 2010) demonstrated the functionality of a scientific mission, a CubeSat-based bistatic radar experiment capable of studying ionospheric disturbances. The scientific payload operated as expected, as demonstrated by a successful science experiment. The majority of the bus systems performed as expected as well, including the GPS-based position and time subsystem, attitude determination and control, communications, and on-board processing. The RAX-1 mission ended prematurely after approximately 60 days of operation due the solar panel failure. This failure has been corrected on the second RAX satellite, RAX-2. 3)4)

RAX-2 will enable scientific experiments to be carried out throughout the planned one-year mission lifetime. The RAX-2 scientific mission goals are similar to those of RAX-1 with expanded opportunities for scientific measurements. These scientific measurements will improve the fundamental understanding of ionospheric field-aligned irregularities.

Spacecraft:

Like its predecessor, the RAX-2 nanosatellite is a 3U (3 unit) standard CubeSat with a volume of 10 cm x 10 cm x 30 cm and a mass of ~ 3kg. The nanosatellite structure uses a Pumpkin kit, which was modified by the project to suit the mission needs. Spacecraft stabilization is performed with passive magnetic control (aligned along one axis with the magnetic field lines).

RAX-2 carries out the same mission concept as RAX-1 with an improved satellite bus. In addition to studying naturally occurring FAI (Field-Aligned Irregularity), the irregularities will be generated on-demand using high powered ionospheric heaters, providing the opportunity for more scientific experiments.

The most significant change between the satellites is the redesign of the RAX-2 solar panels to mitigate the power degradation experienced by RAX-1. There are seven solar cells per panel on RAX-2, whereas there were eight per panel on RAX-1. This was done to ease the fabrication process and to facilitate the use of bypass protection diodes between each individual solar cell.

Another improvement in the RAX-2 design is an increased number of photodiodes (sun sensors). RAX-1 utilized photodiodes mounted on the six orthogonal faces of the satellite. However, due to the limited field of view of each individual photodiode (70º half-angle), this configuration does not yield a complete sun vector for all possible orientations. RAX-2 uses additional photodiodes mounted at angles on the solar panels to increase the sun sensor coverage to nearly 100% of all possible orientations, which will improve the accuracy of the attitude determination system. Although RAX-1 meets the attitude determination requirements, the increase in capability was desired for the general improvement of the attitude determination subsystem.

RF communications: TT&C communications are provided in UHF (UHF transceiver) at 38.4 kbit/s in downlink. The science data are transmitted in S-band (2.4 GHz) at 115.2 kbit/s (MHX 2400 radio). Use of UHF turnstile antenna and S-band patch antenna. The primary link is UHF with the S-band as backup.

The Li-1 (Lithium-1) radio CubeSat Kit of Astronautical Development LLC, Sunnyvale, CA is used for UHF communications. 7)8)

MXL-developed, provides power regulated at 3.3V and 5 V, as well as 7.4 V from the battery. The battery is a 7.4 V, 4.4 Ah Li-Ion. There are four body-mounted solar panels each with 7 Emcore BTJM solar cells producing 7.2 W peak power at 25ºC.

Attitude determination

MXL-developed subsystem that includes magnetometers, photodiodes, and a 3-axis rate gyroscope. Raw sensor data is downloaded for calibration and attitude estimation. Demonstrated accuracies of 2-3º (1σ).

Secondary payloads: The secondary payloads on the NPP mission are part of NASA's ElaNa-3 (Educational Launch of Nanosatellites) initiative. All secondary payloads will be deployed from standard P-PODs (Poly Picosatellite Orbital Deployer). 9)

• M-Cubed (Michigan Multipurpose Minisat), a 1U CubeSat of the University of Michigan, Ann Arbor, MI. M-Cubed features also the collaborative JPL payload called COVE (CubeSat On-board processing Validation Experiment).

Orbit of the secondary payloads: After the deployment of the NPP primary mission, the launch vehicle transfers all secondary payloads into an elliptical orbit for subsequent deployment. This is to meet the CubeSat standard of a 25 year de-orbit lifetime as well as the science requirements of the payloads riding on this rocket. The rocket will take care of the maneuvering and when it reaches the correct orbit, it will deploy all of the secondary payloads, into an orbit of ~ 820 km x ~ 400 km, inclination = 102º.

Status of mission:

• The RAX-2 mission has ended, it successfully met it's mission objectives, providing a wealth of new knowledge relating to the upper ionosphere. The RAX radar echo discovery has convincingly proved that miniature satellites, beyond their role as teaching tools, can provide high caliber measurements for fundamental space weather research. - Besides it's scientific accomplishments, the RAX missions provided significant technological improvements to small satellite subsystems, and trained a generation of engineers, now working throughout academic, government, and industrial institutions. 10)

- The RAX-2 nanosatellite has completed more than 30 conjunction experiments with the Advanced Modular Incoherent Scatter Radar chain of incoherent scatter radars in Alaska and Resolute Bay, Canada. Coherent radar echoing occurred during four of the passes: three when E region electron drifts exceeded the ion acoustic speed threshold and one during HF heating of the ionosphere by the High Frequency Active Auroral Research Program heater. 11)

• July 1, 2013: The project team tracked the RAX-2 nanosatellite at the SRI dish (45 m Ø) during six passes this past week but unfortunately was unable to communicate with the MHX 2400, RAX’s backup radio. The team attempted direct communication with the radio, which if the radio were turned on, should work independently of the flight computer. Given the silence and previous work with the UHF radio, this indicates that both radios are inactive and the silence is caused by an upstream anomaly on the flight computer or power system. 12)

• June 12, 2013: RAX-2 has remained silent on it’s UHF frequency since April 20, 2013. Since there haven’t been any transmissions over UHF, even when listening with high-gain antennas at SRI international, the next step is to attempt to diagnose where the failure has occurred. There is a backup Microhard S-band radio onboard, and the project is attempting communication with that system this month. Although the cease in operations was unexpected, RAX-2 fully completed its planned one-year mission and has provided unprecedented measurements of ionospheric irregularities. 13)

Figure 5: Radar scatter as a function of altitude (y-axis) and aspect angle (contour lines, in degrees) from the March 8, 2012 experiment with PFISR (image credit: UMich)

• On January 18, 2013, RAX-2 carried out a radar experiment with the PFISR (Poker Flat Incoherent Scatter Radar) site, but no echos from FAI were detected. This was the first experiment using GPS time synchronization following GPS testing in December 2012.

• On June 12, 2012, RAX-2 measured radar scatter again! This is the third echo detection in a row, and the first using the RISR (Resolute Incoherent Scatter Radar), located at Resolute, Alberta, Canada. 14)15)

• On May 15, 2012, the RAX-2 nanosatellite was 200 days on orbit. So far, the project received over 152,000 beacons, an achievement that would not have been possible without the help of amateur radio operators around the globe. RAX-2 continues to prove that small satellites can provide high caliber measurements for fundamental space weather research. 16)

- In late May 2012, RAX-2 has been operating for over 215 days and has provided over 100 MB of science engineering data, including 159,000 telemetry packets (beacons). RAX-2 has conducted 19 radar experiments and has provided the highest resolution (in altitude and aspect angle) auroral region UHF radar measurements ever taken. 17)

• In April 2012, RAX-2 is operating nominally. The cumulative sum total data received as of April 16, 2012 is 137,668 beacons and counting (70 MB of data). 18)19)20)

- On April 24, the project received RAX-2 beacons over the MHX 2400 radio! Nine beacons were decoded during the middle of the pass. 21)

- In summary: so far RAX-2 demonstrated new and unique science capabilities.

- The power system is in good health; the solar panel failure experienced with RAX-1 was corrected. The other bus subsystems are performing as expected.

• RAX-2 detects radar echoes on March 8, 2012 (the first detection of scatter from FAI). The experiment was conducted with PFISR, and the resulting RTI is shown in Figure 7. The radar echo from FAI is seen in the boxed region of the plot, just above the direct radar beam. A zoomed-in portion of the portion of the data containing the radar echo is shown in Figure 8. Figure 9 shows FAI altitude, SNR (Signal-to-Noise Ratio), and magnetic aspect angle corresponding to data along the red line of Figure 8. In this experiment, the radar scatter was measured from irregularities at altitudes between 80 km and 115 km (Ref. 20).

This experiment has provided unprecedented characterization of FAI. The irregularities are located with an altitude resolution of 3 km and sub-degree resolution in aspect angle, which is a first for aural region measurements. The measurements from this experiment as well as many future RAX experiments will enable improved characterization of meter-scale ionospheric irregularities (Ref. 20).

Legend to Figure 9: The plot is a fit to the maximum intensity of the radar echo. Radar scatter was caused by FAI between the altitudes of 80 -115 km. The angle shown in the plot is a measure of magnetic-field alignment. The maximum intensity of the scatter signals corresponds to an angle of 90°, where the Bragg wave vector is exactly perpendicular to the geomagnetic field lines.

- Uplinked custom code to try to diagnose the failure. No conclusions on the cause or exact failure.

- The project can communicate with the card, but cannot initialize it, read, or write data.

Implications: cannot execute stored commands, record data to the card, or download data from the card.

Recovery:

- There is 2 kB of available codespace on the FCPU RAM. We upload code to the spacecraft and execute the code. This includes the ability to schedule commands.

- Science data downloads resume on Feb 17, 2012

- Experiments resume on Feb 20,2012.

- Still in development: the ability to store telemetry on-board as well as log GPS data.

The SD failure cause is unknown.

• Dec. 6, 2011: RAX-2 is processing its second science experiment. The project is transitioning into a state of operations where experiments with PFISR will be performed often, and the data download priorities will be for any data collected during space weather activity. 23)

• Nov. 2011: With spacecraft checkout complete, the project transitioned into nominal operations. The main activities within nominal spacecraft operations include scheduling and processing experiments; scheduling other events such as radar receiver noise floor characterization, logging GPS data, and collecting high frequency telemetry data; and down linking data. The project normally uploads a schedule to RAX-2 once per day which includes time-tagged commands for the next 24 hours. When no experiment or related activities are performed, data is downloaded to the University of Michigan and SRI ground stations, as well as the global amateur radio community (Ref. 20).

• Nov. 24, 2011. The GPS receiver is operating successfully, locking to the GPS constellation and providing excellent signal strengths. GPS is necessary to provide accurate time and position to the R2 satellite necessary to accomplish our science goals.

• Nov. 22, 2011. RAX-2 completed its first science radar experiment over the Poker Flat Incoherent Scatter Radar transmitter in Poker Flat, Alaska. The RTI (Range-Time Intensity) plot from this experiment is shown in Figure 10. The RTI plot is the result of on-board processing of the raw radar measurements. This plot shows the intensity of the radar measurements along with time between transmission and receipt of the signals versus time into the experiment. In this plot, no radar scatter was detected from FAI (Field-Aligned Irregularity). This was expected given the calm space weather conditions at the time of the experiment.

Legend to Figure 10: The range delay from transmission to reception is plotted as a function of time for a 300 second experiment. The data looks clean and the radar main beam is clearly visible. No FAI (Field-Aligned Irregularity) echoes are visible, but based on space weather conditions, none were expected.

The RAX mission is a UHF radar experiment to remote sense ionospheric plasma properties. The primary mission goal is to assess the generation and distribution of natural ionospheric irregularities—a space weather phenomenon that can severely degrade the performance of communication and navigation assets. Secondary goals include training the next generation of space engineers and monitoring amateur radio frequency bands.

Legend to Figure 11: The radar pulses are reflected off the magnetically aligned plasma disturbances. The reflections scatter in cones and are received by the RAX satellite. The RAX measurement method is utilizing the bistatic radar configuration. A ground-based ISR (Incoherent Scatter Radar) station illuminates the ionosphere. Radar scatters from the irregularities and is measured by the orbiting radar receiver.

The RAX payload receiver, developed at SRI, operates in a snapshot acquisition mode collecting raw samples at 1 MHz for 300 s over the experimental zone. Following each experiment, the raw data is post-processed for range-time-intensity and Doppler spectrum. The snapshot raw data acquisition enables flexibility in forming different radar pulse shapes and patterns. In addition, the PFISR electronic beam steering capability can be utilized for simultaneous multiple beam position experiments. 24)

The radar receiver is capable of operation with the five UHF ISRs (Incoherent Scatter Radars):

- PFISR (Poker Flat Incoherent Scatter Radar), Poker Flat, AK

- RISR (Resolute Bay Incoherent Scatter Radar), Alberta, Canada

- ESR (EISCAT Svalbard Radar, in Longyearbyen, Spitzbergen, Norway)

- Millstone Hill ISR of MIT in Westford, MA, USA, and

- Arecibo ISR in Puerto Rico.

The receiver can also operate with the MUIR radar located at the HAARP (High Frequency Active Auroral Research Program) facility in Gakona, AK, as part of active experiments. The satellite can perform 2-3 experiments per day. Figure # shows the 1-minute satellite tracks that pass through the scattering zone. There are ~1000 passes good for experiments in the 1-year mission lifetime.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.